The present invention relates to an optical module and, more specifically, an optical module ideal in an application in which two or more light signals with varying wavelengths that are propagated through a single optical fiber are transmitted/received.
In the field of optical communication and particularly in the field of subscriber optical communication, a typical example of which is fiber to the home (FTTH) communication, it is becoming increasingly common to adopt the single-conductor bidirectional communication method. This method enables communication of data and the like by propagating two different light signals with varying wavelengths bidirectionally through a single optical fiber. It achieves higher efficiency in the optical fiber utilization compared to the other method in which bidirectional optical communication is realized by using two optical fibers, one of the optical fibers used for communication along a given direction and the other optical fiber used for communication along the other direction. Such single-conductor bidirectional communication may be achieved by adopting, for instance, a multiple wavelength system whereby the bidirectional optical communication is executed by propagating light signals with varying wavelengths along opposite directions from each other through a single optical fiber. The light signals used in such an application typically have wavelengths of 1.3 μm and 1.5 μm.
In the single-conductor bidirectional communication method described above, a light signal with a transmission wavelength lambda 1 may originate at point A at one end of a single optical fiber and a light signal with wavelength lambda 2 which is different from lambda 1 may originate at point B at the other end of the optical fiber. In this case, the light signal with wavelength lambda 1 transmitted from point A is received at point B, and likewise, the light signal with wavelength lambda 2 transmitted from point B is received at point A. Since the light signals with wavelength lambda 1 and the wavelength lambda 2 are propagated along directions opposite from each other through the optical fiber, a branching filter having a function of identifying and differentiating the individual wavelengths is normally installed at each end of the optical fiber.
FIG. 8 schematically illustrates a structure that may be adopted to implement the method described above. In the example presented in FIG. 8, a branching filter 2a is connected to one end of a single optical fiber 1 on the point A side with a laser diode (hereafter abbreviated to LD) 3a and a photodiode (hereafter abbreviated to PD) 4a connected to the branching filter 2a. In addition, a branching filter 2b is connected at another end of the optical fiber 1 on the point B side and an LD 3b and a PD 4b are connected to the branching filter 2b. Light with wavelength lambda 1 emitted from the LD 3a passes through the branching filter 2a and becomes divided at the branching filter 2b before entering the PD 4b. Likewise, light with wavelength lambda 2 emitted from the LD 3b passes through the branching filter 2b and is divided at the branching filter 2a before entering the PD 4a. 
A transmission/reception module having such a branching function and also having both a light signal transmission function and a light signal reception function integrated therein may be utilized at homes and offices. For this reason, it is crucial to provide a transmission/reception module that can be offered as a compact, inexpensive unit so as to achieve further popularization of optical communication.
Now, in reference to FIG. 9, a transmission/reception module utilized in single-conductor bidirectional optical communication in a first example of the related art is explained (see C-208, p 208 “Receptacle-type Bidirectional Multiple Wavelength Optical Module I” by Masahiro Ogusu et al., the Electronic Information Communication Conference, a Electronics Society, 1996). In this module, a wavelength filter 21 achieving wavelength selectivity is secured inside a rectangular parallelepiped housing 20. In addition, an optical fiber 27, an LD 22 and a PD 23 are secured to an outer back of the housing 20, with an optical fiber lens 24, an LD lens 25 and a PD lens 26 respectively fixed onto the optical fiber 27, the LD 22 and the PD 23.
A light signal with wavelength lambda 1 emitted from the LD 22 is converted to a parallel beam at the lens 25, is reflected by the wavelength filter 21 to change its advancing direction by 90° and is condensed through the optical fiber lens 24 to reach the optical fiber 27. A light signal with wavelength lambda 2 having been propagated through the optical fiber 27, on the other hand, is converted to a parallel beam at the optical fiber lens 24, is transmitted through the wavelength filter 21 and is condensed onto the PD 23 via the PD lens 26. The functions as a transmission/reception module for single-conductor bidirectional optical communication are achieved by adopting the structure described above.
In reference to FIG. 10, a transmission/reception module for single-conductor bidirectional optical communication achieved in a second example of the related art is explained (see Japanese Laid Open Patent Publication No. H 11-218651). This module, which includes a V-grooved substrate 52 having an optical guide 51 constituted of quartz, adopts the following structure. A V-shaped groove 53 is provided at one end on the V-grooved substrate 52, and an optical fiber 54 is secured onto the V-shaped groove 53. A diagonal slit 55 constituted of a narrow diagonal groove is provided halfway through the optical guide 51. A wavelength filter 56 is inserted at the diagonal slit 55. A hole 57 through which a light signal can travel is formed at the substrate directly under the wavelength filter 56. A PD 58 is disposed over an area of the substrate rear surface that connects with the hole 57, and an LD 59 is set at the trailing end portion of the optical guide 51.
Another priority art literature related to the present invention is Japanese Laid Open Patent Publication No. 2002-328204.